A load circuit mounted on a vehicle supplies electric power to loads such as a bulb and a motor. This load circuit includes a battery and semiconductor switches (MOSFETs or the like) provided between the battery concerned and the loads. The battery, the semiconductor switches and the loads are connected to one another through conductors including electric wires. In the load circuit, a control circuit that outputs drive/stop signals for switching ON/OFF the semiconductor switches is further provided. By the drive/stop signals, the loads are switched between driving and stopping.
In the above-described load circuit, fuses are provided. The fuses break the circuit instantaneously when an overcurrent flows through the loads, and thereby protects the loads, the electric wires, the semiconductor switches and the like (refer to Patent Citation 1).
FIG. 1 is an explanatory view schematically showing a conventional load circuit. Power supply-side terminals 101 are connected to a battery VB through an automotive electronic control unit (ECU) 102 and a junction box (J/B) 103.
In the ECU 102, a plurality of semiconductor switches Tr1 such as MOSFETs are provided. ON/OFF of the semiconductor switches Tr1 are controlled by a control IC 104. Fuses F1 are provided upstream of the respective semiconductor switches Tr1. The fuses F1 protect electric wires W101 provided downstream of the fuses F1 concerned. In other words, the electric wires W101 have an electric wire diameter (cross-sectional area) enough to withstand a breaking current of the fuses F1. Moreover, the semiconductor switches Tr1 provided downstream of the fuses F1 have characteristics of being capable of withstanding the breaking current of the fuses F1.
Moreover, in the J/B 103, fuses F2 are provided. The fuses F2 protect an electric wire W102 provided downstream of the fuses F2. In other words, the electric wires W102 have a diameter (cross-sectional area) enough to withstand a breaking current of the fuses F2.
Here, for example, when bulbs are used as the loads 101, there is a possibility that the fuses F1 and F2 may be deteriorated owing to rush currents generated when the bulbs are turned ON, and to repetition of ON/OFF the bulbs. Therefore, in some case, erroneous breakdown of the fuses F1 and F2 occurs, which results from the deterioration of the fuses F1 and F2. Here, the deterioration is caused because the fuses F1 and F2 are used for years. In order to prevent such a trouble, fuses for which a margin for a load current is considered are selected. Specifically, fuses are used, in which the breaking current is set somewhat higher than usual. As a result, it is necessary to use electric wires, semiconductor switches and the like, which are adapted to characteristics of the fuses for which the margin is considered, and it has become difficult to miniaturize the electric wires and the semiconductor switches, which are used for the load circuit.